1. Technical Field
The present invention relates to photovoltaic devices, and more particularly to contact structures which improve performance of heterojunction cells.
2. Description of the Related Art
Heterojunction with intrinsic thin layer (HIT) solar cells have improved in efficiency (e.g., 23% efficiency in the laboratory and 21% efficiency in production). The HIT cells are comprised of intrinsic/doped hydrogenated amorphous silicon (a-Si:H) serving as front (emitter) and back contacts, on a crystalline silicon (c-Si) absorber with p-type or n-type doping. An advantage of HIT cells is the low deposition temperature of a-Si:H (˜200° C.) which offers a lower thermal budget as compared to conventional c-Si cell processes (˜1000° C.). The low process temperature also permits the use of low-cost Si wafers by preserving carrier lifetime.
Referring to FIG. 1, an energy band diagram of a conventional HIT cell 10 on a p-type crystalline silicon (c-Si) substrate 12 is shown in equilibrium. The equilibrium Fermi level is denoted by EF, and the conduction band and valence band edges are denoted by Ec and Ev, respectively. Open circuit voltage of the cell 10 is the difference of the quasi Fermi level for electrons at an emitter 14 (the equilibrium Fermi level in n+ a-Si:H layer 16) and the quasi Fermi level for holes at a back contact 18 (the equilibrium Fermi level in p+ a-Si:H layer 20). The emitter 14 includes a front contact 15. Intrinsic layer 22 is disposed between layer 16 and substrate 12.
The band offsets and equilibrium Fermi level parameters are the fundamental material properties of a-Si:H and may only vary marginally by changing the growth conditions. For high-quality a-Si:H, the measured values of ΔEc and ΔEc are in the range of 0.1-0.2 eV and 0.4-0.5 eV, respectively. The equilibrium Fermi level cannot move closer to the conduction band than 0.15-0.2 eV in n+ a-Si:H 16, and cannot move closer than 0.4-0.45 eV to the valence band in p+ a-Si:H 20 by increasing the doping concentration. This is because doping incorporation increases the defect density in a-Si:H and eventually pins the Fermi level position.
Replacing a-Si:H with other compounds results in creating larger band-offsets. Such band-offsets may improve the open circuit voltage, but at the cost of reducing fill factor (FF). This is because the tunneling rate of majority carriers is lower through the larger band-offsets. In addition, the issue of low doping efficiency applies to these compounds as well.